LT3999
5
3999fa
For more information www.linear.com/LT3999
TYPICAL PERFORMANCE CHARACTERISTICS
Soft-Start (ILIM/SS) Current Switch Duty Cycle
PIN FUNCTIONS
SWA, SWB (Pin 1, Pin 10): SWA and SWB pins are the
open-collector nodes of the power switches. These pins
drive the transformer and are connected to the outer ter-
minals of the center tapped transformer. Large currents
flow through these pins so keep PCB traces short and wide.
RBIAS (Pin 2): The RBIAS pin sets the bias current of
the power switches (SWA and SWB). Connect
the pin to
a 49.9k resistor to GND.
V
IN
(Pin 3): The V
IN
pin is the main supply pin for the
switch driver and internal regulator. Short duration, high
current pulses are produced during the turn on and turn
off of the power switches. Connect a low ESR capacitor
of 4.7µF or greater.
UVLO (Pin 4): The UVLO pin has a precision threshold
with hysteresis to implement
an accurate V
IN
undervolt-
age lockout. The UVLO function disables switching and
sets the part into a low current shutdown mode. Connect
the UVLO pin directly to V
IN
or to a resistor divider string.
OVLO/DC (Pin 5): The OVLO/DC pin has a precision thresh-
old with hysteresis to implement an accurate V
IN
overvolt-
age lockout. The OVLO function disables the switching.
Connect OVLO/DC
pin to ground to disable the function
or to a resistor divider string to program the duty cycle.
RDC (Pin 6): The RDC pin is the duty cycle control pin. A
resistor to ground sets the duty cycle. If unused leave the
pin floating or connect to the OVLO/DC pin.
RT (Pin 7): The RT pin sets the switching frequency of
the power switches.
SYNC (Pin 8):
The SYNC pin synchronizes the part to an
external clock. Set the internal oscillator frequency below
the external clock frequency. Synchronizing the clock to
an external reference is useful for creating more stable
positioning of the switcher voltage or current harmonics.
Connect the SYNC pin to ground if not used.
ILIM/SS (Pin 9): The ILIM/SS pin sets a threshold level
for the cycle by cycle maximum
switch current. Imple-
ment soft-start with a capacitor, C
SS
, placed on this pin to
ground. An internal current source charges the capacitor.
The R
ILIM
, C
SS
time constant sets the soft-start time and
ramps the maximum switch current threshold at start-up.
If the ILIM/SS function is not used, float this pin and the
current limit will default to the internal limit.
GND (
Pin 11): The ground pin is the exposed pad of the
package. Solder the exposed pad directly to the ground
plane.
TEMPERATURE (°C)
–50
4
SOFT-START CURRENT (µA)
5
7
8
9
14
11
0
50
75
3999 G09
6
12
13
10
–25 25
100
125
150
TEMPERATURE (°C)
–50
10
DUTY CYCLE (%)
15
20
25
30
0 50
100
150
3999 G10
35
40
–25 25
75
125
